The invention relates to apparatus for mixing or blending developer mix particles consisting of carrier and toner in a developer station of a non-mechanical printing or copying machine.
Typically non-mechanical printing or copying machines function according to electrophotographic principles, wherein electrostatic latent images of characters to be printed are generated on a recording medium, such as a photoconductive drum. The drum has on it a semi-conductor layer of photo-electrical or di-electrical material on which electrostatic charge images of the characters to be printed or copied are generated. These electrostatic images are subsequently inked with a toner powder, which is typically black, in a developer station. The toner images are subsequently transferred to sheet paper on which they are fixed. The developer station typically includes at least one developer unit generally referred to in the art as a magnetic brush developer. The magnetic brush developer, as a rule, contains a magnetic brush or drum mounted for rotation so as to continually bring particle developer mix, consisting of iron carrier particles and toner powder particles, into contact with the electrostatic images recorded on the photoconductive drum surface. The charge images are inked by adherence of toner particles to the charge images, due to electrostatic forces. The general principles of developer station construction are known in the art as shown, for example, in U.S. Pat. Nos. 3,784,297 and 3,883,240.
During developer operation, toner is continuously removed from the developer station by adherence to the character charge images. Thus, the concentration of toner in the developer mix, that is the percentage of toner particles to carrier particles, is constantly decreasing in the developer station and new toner must be supplied to the station in order to make up for that which is lost. It is necessary, in order to maintain a lever of inking quality in the developer mix, to enable to newly supplied toner to uniformly mix with the toner-depleted body of mix. One known mixing device is disclosed in U.S. Pat. Nos. 3,784,297 and 3,883,240. The mixing device comprises two mixing screws disposed side-by-side in a developer station separated from one another by a partition wall. The mixing screws are arranged to convey particles in opposite directions. Developer mix falling back into the developer station from the top of the developer or magnetic drum after being conducted to the charge image surface of the printing or copying machine photoconductive drum drops back into the area of the two mixing screws. Particle flow along the mixing screws is serial with the outer screw feeding its conveyed particles to the inner screw disposed adjacent the developer drum. New toner particles are received from a reservoir hopper by the outer screw such that the screws serve to blend the newly supplied toner into the developer mix body. An opening in the partition wall enables the particles from the outer screw to be conveyed to the inner screw.
An object of the present invention is to provide apparatus in a developer station by which two mixing screws are arranged to provide a high level of mixing or blending of developer mix and new toner particles in high-speed printing and copying machines.